Topography link and/or wavefront guided custom ablation is a newly developed technology aiming to achieve supernormal visual acuity through photorefractive surgery. In a custom-ablation photorefractive surgical procedure, a computer of the surgical system reads in the patient's data from a topography or wavefront device and controls the scan of a surgical laser beam to generate a customized ablation profile on the subject's cornea. It can thus remove corneal irregularity and correct low and high-order optical aberrations of the subject's eye.
It is generally expected that the patient's visual acuity, contrast sensitivity and visual function would be significantly improved once the refractive prescription and any irregularity, or high-order aberrations are removed. These irregularities and high-order aberrations of the subject's eye can be measured objectively by a corneal topographer or ophthalmic wavefront instrument. As the diagnostic instrumentation and the refractive surgery procedures themselves have been refined, however, there has been a controversy with respect to the results that have been observed. It has been found clinically that people with excellent visual acuity, such as jet-fighter pilots, may have high-order aberrations not significantly different from normal eyes, while patients who have undergone a reduction in their high-order aberrations may have similar visual acuity as normal eyes.
It is understood that conventional customized ablation is based on objective diagnostic data of the eye. Conventional customized ablation attempts to make the patient's eye into an aberration-free optical system. On the other hand, visual acuity is rather a subjective phenomenon, involving image recognizing and processing by the human brain. The neurological processes involved in constructing a visual image are expected to vary between individuals. Therefore, an aberration-free eye may not necessarily produce optimal visual acuity and function in a given patient, and it is thereby conceivable that the optimal visual acuity may be attained by an eye not free of aberrations. This has been the clinical experience. It is well known that the lower order aberrations (refractive sphere and cylinder) as determined objectively with instrumentation needs to be refined clinically, using the patient's subjective response, to allow the patient to choose the required optical prescription that satisfies the entire optical system, comprised of the eye and the brain. Any customized refractive surgical procedure that uses objective data without the subjective participation by the patient may thus be less than ideal.